Some existing approaches to introducing atoms and ions (e.g., alkali atoms and ions) into an environment from a source of same involve: heated chemical reactions using, for example, alkali sources sold by SAES Getters S.p.A. of Milan, Italy (“SAES”) or Alvatec Alkali Vacuum Technologies GmbH of Althofen, Austria (“Alvatec”); using solid state atom sources; pulsed laser ablation of metal in a vacuum system; and using ovens filled with alkali metal. One example of a solid state atom source is described in U.S. Pat. No. 8,999,123. These exemplary atom and ion sources may be used to provide a controlled partial pressure of atoms in atomic sensor systems, such as atomic clocks, atomic magnetometers, and cold atom inertial systems (e.g., gyroscopes and accelerometers). Moreover, ion beams may be used to provide thrust for spacecraft, in ion beam etching, and to source ions to ion-traps for atomic sensors.
The exemplary approaches for producing atoms and ions described above suffer, however, from several drawbacks. For example, the products sold by SAES and Alvatec typically draw large amounts of current and power, are heated to a high temperature, and produce magnetic fields (from the high currents), which are all undesirable traits for most atomic sensors. In addition, undesirable gasses are produced as a by-product of use with some of these alkali sources.
Existing solid state atom sources often use shadow masked, evaporated electrodes that are inefficient due to large line-width and a low density of triple-phase boundaries (TPBs)—i.e., a low density of regions where a body of the solid state ion-conducting atom source, electrodes, and an environment into which the atoms are to be released or from which atoms are to be absorbed meet. Currently, shadow masking of electrodes can only produce wide metal fingers having widths of at least about 100 micrometers or more that trap most of the atoms to be produced in the body of the solid state atom source below the electrodes, resulting in low current efficiency. The resulting current conversion efficiency based on atoms produced per electron flowing through the system is generally less than 1%. Narrow slot shadow masks can be fabricated by, e.g., laser machining of sheet metal, or using microelectromechanical systems (MEMS) etching techniques. However, these techniques typically do not achieve interpenetration of the metal and ion-conducting phases. As such, adhesion of the metal electrodes is not optimized.
FIG. 1 shows an exemplary prior art system 10 including a solid state source 12 with metal finger electrodes 14 produced by shadow masking. The source 12 also includes a copper contact 16 that is connected to the metal finger electrodes 14 and can be connected to a voltage source. The width of the narrow metal electrode finger electrodes 14 is limited to about 130 micrometers (130 μm) by available shadow mask technology. The pictured source 12 has a diameter D of 12 millimeters (12 mm).
FIG. 2 is a schematic cross-sectional view of a system 10 as shown in FIG. 1. FIG. 2 shows a metal finger electrode 14, which serves as a cathode. The metal finger electrode has a width of wm. The metal finger electrode 14 extends over a solid state ionic conductor 18, which extends over an anode 20. In operation, mobile ions, such as rubidium (Rb) ions within the solid state ionic conductor 18, migrate towards the metal finger electrode 14 under the influence of an electric field, and accumulate in a region 22 adjacent to the metal finger electrode 14 where they are neutralized. The region 22 extends beyond the edges of the metal finger electrode 14 by a distance of tsc on each side of the metal finger electrode 14. Atoms that arrive near the edge of the metal finger electrode 14 can evaporate into the surrounding environment, which is often a vacuum. These edge regions form TPBs 24 where the electron conductor (in this case, the metal finger electrode 14), ionic conductor 20, and empty space, also referred to herein as voids or pores, meet. Most of the mobile atoms arrive under the wide metal finger electrode 14 and are trapped due to limits on the width of the metal finger electrode 14. Furthermore, a continuous layer of the alkali element (e.g., Rb, Cs) under the metal finger electrode 14 can result in failure of the system 10, particularly if it is exposed to air and/or moisture during or after operation.
An alternative to forming metal finger electrodes on the surface of an ionic conductor with a shadow mask to fabricate an ion/atom supply/sink system is to use photolithography to create narrower interconnected lines by liftoff and/or etching. However, these surface lines generally still suffer from poor adhesion to the ionic conductor. Furthermore, many fast ionic conductors are incompatible with photoresist and developer chemistries or with processes including photolithography for defining electrodes on surfaces of the ionic conductors, since the ionic conductors, such as ceramic ion conductors, are hygroscopic.
Pulsed laser ablation typically requires the addition of a high power pulsed laser to the system, and oven sources generally consume power and cannot be easily switched on and off.
Accordingly, there is a need for an improved electrode system for solid state atom and ion sources and sinks.